Palaeozoic acritarchs mostly represent organic-walled cysts of marine phytoplankton, and therefore, as primary producers, played an important role in the evolution of marine ecosystems. In this study, we use a selection of the most abundant acritarch taxa from the Cambrian and Ordovician of China to understand the evolution of the palaeoecological patterns of the phytoplankton over the period. The taxa are attributed to 40 easily distinguishable morphotypes, of which the precise palaeoenvironmental distribution from 60 localities is available. By placing the 40 morphotypes on inshore–offshore transects it can be concluded that acritarch microfloras were limited to inshore environments during the early Cambrian, and progressively extended from inshore environments to offshore marine habitats during the later parts of the Cambrian and towards the Early Ordovician, with a prominent shift near the Cambrian–Ordovician boundary, confirming the onset of the ‘Ordovician plankton revolution’. In addition, the acritarch morphotypes evolved from low-diversity assemblages in the early Cambrian, dominated by simple spherical forms with limited ornamentation and simple process structures, to highly diverse assemblages with very complex morphologies in the Early and Middle Ordovician. During the Ordovician, the complex acritarch assemblages occupied most marine habitats, with palaeoecological distribution patterns similar to modern dinoflagellates. This article is part of the theme issue ‘The impact of Chinese palaeontology on evolutionary research’.
Abstract China presently comprises several independent tectonic palaeoplates or terranes and parts of other blocks, which have been assembled over geological time. In the Ordovician, these blocks included South China, North China, Tarim, Qaidam, Junggar, Qiangtang-Qamdo, Lhasa and partially Himalaya, Sibumasu and Indochina, as well as the Altay-Xing'an and Songpan-Garze fold belts, which were discrete but near-adjacent. Twelve stratigraphic megaregions bounded by tectonic sutures or major fault zones can be recognized. Some of them are further differentiated into several regions according to the lithological and biotic facies or distinct stratigraphic sequences. Here, the palaeontologic features and biostratigraphic framework of these stratigraphic megaregions and regions are summarized. The unified biostratigraphic framework presented herein is supported by 33 graptolite biozones and 27 conodont biozones, together with supplementary biozones, communities or associations of brachiopods, trilobites, cephalopods, chitinozoans, acritarchs and radiolarians. With constraints of integrative chronostratigraphy, biostratigraphy, chemostratigraphy, cyclostratigraphy and magnetostratigraphy, along with some geochronologic data, our understanding of the temporal and spatial distribution of the Ordovician lithostratigraphic units on these major blocks has been significantly advanced. Vast amounts of new data accumulated in recent decades also constrain the major Ordovician geological and biotic events evident in China, such as marine anoxia, faunal turnovers and tectonic orogenies.
First Appearance Datums (FADs) of selected, easily recognizable acritarch morphotypes are assessed to determine their potential contribution to correlation between Lower and Middle Ordovician stages and substage divisions along the Gondwanan margin (Perigondwana) and between Perigondwana and other palaeocontinents. The FADs for 19 genera, species and species groups are recorded throughout their biogeographical ranges. The taxa investigated fall into three groups. Some have FADs at about the same level throughout their biogeographical ranges and are useful for long-distance and intercontinental correlation. Among these are Coryphidium, Dactylofusa velifera, Peteinosphaeridium and Rhopaliophora in the upper Tremadocian Stage; Arbusculidium filamentosum, Aureotesta clathrata simplex and Coryphidium bohemicum in the lower–middle Floian Stage; Dicrodiacrodium in the upper Floian Stage; Frankea in the Dapingian–lower Darriwilian stages; and Orthosphaeridium spp., with FADs in the Dapingian–lower Darriwilian stages of Perigondwanan regions and at about the same level in Baltica. Other taxa, however, have diachronous (or apparently diachronous) FADs, and this needs to be taken into account when using them for correlation. A second group of genera and species, comprising Striatotheca, the Veryhachium lairdii group and the V. trispinosum group, have a recurring pattern of FADs in the Tremadocian Stage on Avalonia and in South Gondwana and West Gondwana, but in the Floian Stage of South China and East Gondwana. The third group, consisting of Arkonia, Ampullula, Barakella, Dasydorus, Liliosphaeridium and Sacculidium, have FADs that are markedly diachronous throughout their biogeographical ranges, although the global FADs of Arkonia, Ampullula, Liliosphaeridium and Sacculidium are apparently in South China and/or East Gondwana. It is possible that diachronous FADs are only apparent and an artefact of sampling. Nevertheless, an alternative interpretation, suggested by recurring patterns, is that some as yet undetermined factor controlled a slower biogeographical spread over time, resulting in diachroneity.
New palynological studies from the Xiaoyangqiao section (Jilin Province, north-east China), which has been selected as an Auxiliary Boundary Stratigraphic Section and Point (ASSP) for the base of the Ordovician, confirms the presence of Goniomorpha Yin 1986 Yin LM. 1986. Acritarchs. In: Chen JY, editor. Aspects of Cambrian-Ordovician Boundary in Dayangcha, China. Beijing (China): China Prospect Publishing House; p. 314–373. [Google Scholar] in intervals ranging from the late Cambrian (Furongian) to the Early Ordovician (Tremadocian), and allow us to clarify its morphology and affinities. Despite being compared originally to vesicular microfossils with processes arising from one pole, Goniomorpha lacks an enclosed cavity, so it cannot be classified as an acritarch in the strict sense. Instead, we find that the specimens originally described as Goniomorpha are actually part of a wider spectrum of morphologies that are identifiable as the pharyngeal teeth of priapulid worms. The best-preserved specimens resolve the 'processes' as denticles arising from an arch on the margins of a cuticular pad, sometimes with an extending spur and polygonal microstructure. Closely equivalent forms have been reported from Cambrian assemblages of small carbonaceous fossils (SCFs) and in situ on Burgess Shale specimens of the priapulid Ottoia. The findings from the Xiaoyangqiao section extend the taphonomic range of Cambrian-type SCFs into the Ordovician, and provide the first evidence for priapulids from the Ordovician of northern China. More generally, our results demonstrate the reciprocal insights from palynological and SCF-type processing for identifying problematic microfossils.
The present paper reviews all investigations of Silurian acritarchs in China. Since Silurian acritarchs were first reported in China in 1978, significant progress has been made in Chinese Silurian acritarch research. A total of 30 scientific articles and theses have been published on Silurian acritarchs from China. The majority of these papers recorded acritarchs from South China, and most Silurian acritarch studies concentrated on the Llandovery. Two papers describing material thought to be Silurian have since been shown to be Ordovician acritarch assemblages. We have updated the systematics of the taxa in these reported assemblages and documented their biostratigraphic distribution within China. Investigations of Silurian acritarchs, however, are still very limited, and more research is needed for systematic, biostratigraphic, and palaeoecological interpretations.
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